Updating ParamConverterInterface's type hinting

Update ParamConverterInterface definition to match changes to type hinting from the following commit: 

https://github.com/sensiolabs/SensioFrameworkExtraBundle/commit/79ce337d9d1f1592eb276e113a5c1ebad68a2e70
minor #273 Updating ParamConverterInterface's type hinting (jessem)

This PR was merged into the 3.0.x-dev branch.

Discussion
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Updating ParamConverterInterface's type hinting

Update ParamConverterInterface definition to match changes to type hinting from the following commit:

https://github.com/sensiolabs/SensioFrameworkExtraBundle/commit/79ce337d9d1f1592eb276e113a5c1ebad68a2e70

Commits
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c9e84a3 Updating ParamConverterInterface's type hinting